Markham Web Content Viewer (JSR 286)

Giant Hogweed

Giant Hogweed is an invasive, non-native plant that poses a serious threat to human health and natural ecosystems.

Do Not Touch This Plant: If you come in contact with this plant, you could experience severe burns to your skin, and risk temporary or permanent blindness.

Giant Hogweed has been identified in some areas of Thornhill and Markham. Efforts have been made to safely remove the plants; however, caution should still be taken in the infested areas. Please respect any warning signs posted and stay away from the plant.

Health Risk

The clear sap found in the hairs, leaves and stem of the Giant Hogweed plant contains compounds that cause photodermatitis (symptoms can range from redness and itching to painful blistering).

Contact with the eyes can cause temporary or even permanent blindness in some cases.

Key identifying features

Stems: hollow, ridged with red-purple splotching and coarse white hairs, with a diameter of 4 to 10 cm at its base.

Flowers: white, compound umbel (80 or more cm across) made up of 4 to 12 smaller (14 to 40 cm) flat, round units.

Infested Areas

In Thornhill, Giant Hogweed plants have been found by the banks of the East Don River, near the intersection of Bayview and Steeles Avenues continuing upstream to the north-west as far as Yonge Street. Three parks, Almond, Proctor and Woodland, are included in this area.

In Markham, a single stand of Giant Hogweed has been found south of 16th Avenue, a short distance east of Main Street Markham, in an inaccessible natural area.